Optical information storing system

ABSTRACT

An optical information storing system comprises a thin photoconductive film and a thin film of a mixture of nematic liquid crystal and dichroic dye, placed side by side between two transparent electrodes. A direct voltage V is applied between said electrodes. The data to be stored are projected optically onto the photoconductive film with a radiation to which the photoconductor is sensitive. The mixture film is illuminated by an auxiliary radiation which is rectilinearly polarized and has a wavelength corresponding to an optical absorption line of the dichroic dye, the photoconductor being sensitive to this radiation.

United States Patent Assouline et a1.

[54] OPTICAL INFORMATION STORING SYSTEM [72] Inventors: GeorgeAssouline; Eugene Leiba; Erich Spitz, all of Paris, France [73]Assignee: Thomson-CSF [22] Filed: June 23, 1970 [21] Appl. No.: 49,012

[30] Foreign Application Priority Data July 2, 1969 France ..6922335[52] U.S. CI ..350/150, 350/154, 350/155, 350/160, 340/173 LM [51] Int.Cl. ..G02f 1/18 [58] Field ofSearch ..350/160, 150, 154, 155; 1 340/173LM [56] References Cited UNITED STATES PATENTS 3,551,026 12/1970Heilmeier ..350/150 3,064,134 11/1962 Kell .350/160 P 3,475,736 10/1969Kurtz ...350/l50 D.C SUPPL SPU RC1.

[451 May 16, 1972 FOREIGN PATENTS OR APPLICATIONS 1,123,117 8/1968England ..350/l60 OTHER PUBLICATIONS Liquid Crystals & theirApplications, by Fergason et a1, Electro-Technology, l/70, pp. 41-50.

Liquid crystal Imag'e lntensifier & Recorder," B. Kazan, IBM Tech.Disc]. Bul., Vol. 12, No.6, 11/69 pp. 864- 865.

Primary E.\'aminerRonald L. Wibert Assistant Examiner-Jeff RothenbergAtlorney-Cushman, Darby & Cushman [57] ABSTRACT An optical informationstoring system comprises a thin photoconductive film and a thin film ofa mixture of-nematic liquid crystal and dichroic dye, placed side byside between two transparent electrodes. A direct voltage V is appliedbetween said electrodes. The data to be stored are projected opticallyonto the photoconductive film with a radiation to which thephotoconductor is sensitive. The mixture film is illuminated by anauxiliary radiation which is rectilinearly polarized and has awavelength corresponding to an optical absorption line of the dichroicdye, the photoconductor being sensitive to this radiation.

4 Claims, 7 Drawing Figures HEATING DEVICE AUXlLlAR LlGTH SOURCEPATENTEDHAHS I972 3,663,086

saw 1 [1F 2 PATENTEDHAY 16 I972 SHEET 2 BF 2 AUXILIAR UGTH SOURCEOPTICAL INFORMATION STORING SYSTEM The present invention relates to adevice for storing optical information, which utilizes liquid crystals.

Liquid crystals are substances which, within a certain temperaturerange, exhibit, when forming a thin film, special arrangements ofelongated molecules depending upon whether they belong to one of thefollowing classes smectic, cholesteric or nematic.

In the nematic structure, which is concerned here, the molecules arearranged parallel to one another. The medium obtained is thus opticallyanisotropic. Moreover, the orientation of the molecules, and thereforethe direction of the optical axis, can be modified by the application ofan electric field.

The molecules of a dichroic substance, added to a nematic substance,align themselves parallel to the molecules of the nematic substance. Theresulting medium is thus itself dichroic and this property can bemodified by the application of an electric field. A suitably polarizedlight wave will be absorbed by it. If then an electric field (in theorder of V/cm in the case of a layer having a thickness of about 10microns) is applied in the direction of propagation of the light, theorientation of the molecules will change and the wave will cease to beabsorbed. A variable transparency medium has thus been created. Thisphenomenon is described in greater detail in a paper by GE. I-Ieilmeierand LA. Zanoni in Applied Physics Letters, vol. 13, no: 3, August 1,1968, pages 91 and 92, which article is entitled Guest-hostinter-actions in nematic liquid crystals. A new electro-optic effect.

It is an object of the invention to exploit this phenomenon with theview of creating a storage device for storing and reconstituting inoptical form, information which is supplied to it in this form.

According to the invention there is provided an optical informationstoring system comprising a first constant thickness layer of a mixtureof nematic liquid crystal and of a dichroic substance, said substanceexhibiting at least one optical absorption line with respect torectilinearly polarized light a second constant thickness layer ofphotoconductive material located against said first layer; twotransparent electrodes for enclosing said layers first means forcoupling said electrode to a dc. supply source optical means forprojecting said optical information onto said photoconductive layer andsecond means for illuminating substantially uniformly said first layerby a beam of polarized light comprising at least a light radiationhaving a wavelength substantially equal to the wavelength of saidabsorption line.

For a better understanding of the invention and to show how the same maybe carried into effect, reference will be made to the drawingsaccompanying the ensuing description and in which FIGS. 1a and lb areexplanatory diagrams of the phenomenon exploited FIGS. 2a and 2b areexplanatory graphs FIG. 3 schematically illustrates a memory element inaccordance with the invention FIG. 4 illustrates a variant embodiment ofthe element shown in FIG. 3 and FIG. 5 illustrates an embodiment of theaccordance with the invention.

As is known, a dichroic substance is a substance whose opticalabsorption spectrum of rectilinearly polarized light radiation, isclosely dependent upon the molecular orientation of said substance inrelation to the direction of polarization of said light radiation. Forexample, where the directions of orientation of the molecules and thatof the light polarization are parallel to one another, one or moreabsorption lines are obtained at wavelengths within the opticalabsorption spectrum of the substance, which lines substantiallydisappear if the direction of polarization of the light is madeperpendicular to the direction of molecular orientation. This propertyhas been used to obtain a novel electro-optical effect by acting uponthe orientation of the molecules of dichroic substances through themedium of molecules of other substances with which said dichroicmolecules are mixed, said other submemory device in stances being suchthat their molecules can be orientated in controlled fashion by theaction of an electric field, namely molecules of liquid crystals in thenematic state. This phenomenon has been described in more detail in theaforedescribed article and requires no detailed description here.

Let the device shown in section and on a much enlarged scale in FIG. 1abe considered.

Between two transparent electrodes 1, connected to terminals 2, isformed a thin layer of a mixture of molecules of nematic liquid crystal4 and molecules of dichroic substance 3. The arrangement is illuminatedby a beam of rectilinearly polarized light whose direction ofpolarization, illustrated by the vector 5, is parallel to the plane ofthe electrodes 1.

In the absence of any voltage applied between the electrodes l, themolecules of liquid crystal have their longitudinal axes substantiallyparallel to one another and to the plane of the electrodes. In thesecircumstances, the graph plotting the absorption coefficient A of themixture as a function of the wavelength of the radiation propagatingthrough said mixture, has substantially the form shown in FIG. 2a. Thereis an absorption line at the wavelength M, which wavelength depends uponthe dichroic substance used. If, now, a dc. voltage source 6 is arrangedbetween the terminals 2 in order to apply to the mixture an electricfield in the order of some tens of kV/cm (FIG. 1b), then the molecules 4of the liquid crystal will all align themselves perpendicularly to theelectrodes 1 producing a similar orientation in the molecules 3 of thedichroic substance.

Under these conditions, the absorption curve has substantially the formshown in FIG. 2b. The absorption line at the wavelength A, has virtuallydisappeared and becomes less and less obvious the higher the appliedfield strength.

By utilizing PN butoxybenzoic acid as the nematic liquid crystal, andmethyl red as the dichroic colorant in a solution of 1 percent by weightin the acid, an absorption line is obtained at A, 4,825 A. and theapplication of a field in the order of 40 kV/cm changes the color of themixture from reddishorange to yellow if the system is observed bytransmission using polarized white light as the illuminating source.

In accordance with the invention, this phenomenon is exploited in orderto create an optical storage device illustrated in section in FIG. 3.Between two flat, parallel and transparent electrodes 7, there arearranged a thin film 8 of a photoconductive material and a thin film 9of the mixture of liquid crystal and dichroic substance. Thephotoconductor may be of the kind used in vidicon tubes (selenium,antimony trisulphide, lead oxide) or gallium arsenide, lead sulphide,indium antimonide and so on, and is to be selected so that it issensitive to the wavelength A, of the dichroic substance used. The film9 is applied to a smooth face of the photoconductive film 8 and has athickness in the order of 10 p..

The thickness of the film 8 is selected in a manner which will bedescribed later on. Spacers 10, for example of polytetrafluoroethylene(e.g.TEFLON), provide appropriate spacing between the electrode and thefilm 8. The capillary forces suffice to keep the liquid crystal layerand the dichroic substance in position whatever the attitude of thearrangement.

A dc. voltage V is applied between the terminals 11 connected to theelectrodes 7. The film 9 is illuminated by an auxiliary beam 13 of lighthaving a wavelength A, polarized rectilinearly and parallel to theplanes of the electrodes 7. Under the effect of the illumination of aregion P of the photoconductive film 8 by a radiation 12 to which thephotoconductor is sensitive, the corresponding region l of the film 9becomes transparent and an observer will see this change in transparencyin a manner which will be explained in greater detail hereinafter.

The operation of the system is as follows The wavelength of thepolarized radiation 13 is selected to be substantially equal to A thiscorresponding to the absorption line of the dichroic substance used(FIG. 2a).

If a voltage V is applied between the terminals 11, and calling k theratio of the respective impedances of the films 8 and 9 for a givenillumination of the photoconductive film, then the voltage between thetwo faces of the film 9 will be equal to V/(k+1).

If the photoconductive film is not illuminated, the ratio k has a valuek,. If the region P of the photoconductor is illuminated by a radiation12 of given intensity, the ratio k in this region will change to k Thethickness of the photoconductive film 8 and the voltage V are selectedin such fashion that, in the absence of any illumination of the film 8(ratio of the impedances of films 8 and 9 equal to k,), the fieldapplied to the mixture of liquid crystal and dichroic substance issufficiently weak for the absorption line of FIG. 2a to persist, whileon the other hand, for an illuminated zone P (ratio of the impedancesequal to k the field applied to the mixture is sufficiently strong toerase substantially said line (FIG. 2b).

Under these circumstances, if the photoconductor is not illuminated, thebeam 13 will be absorbed by the mixture of liquid crystal and dichroicsubstance. If a region P of the photoconductor is illuminated by theradiation 12, the corresponding region P of the film 9 will becomesubstantially transparent to the beam 13. Then, that portion of the beam13 which passes through the region P,, will ultimately reach thephotoconductive film 8. Because of the sudden variation in therefractive index between the films 8 and 9, part of this portion of thebeam is reflected by the metal layer 8 back to the observer whereas theother part is absorbed by the region P of the photoconductive film 8 andserves to keep this region conductive (k kept at the value k even if theillumination 12 is cut off.

The region P thus appears bright and its brightness is maintained by thelight beam 13. The light information transmitted by the beam 12 is thusstored and is available until the illumination 13 is cut off or thevoltage V likewise, this enabling the erasing of all the storedinformation.

By way of example, if the above-indicated mixture of PN butoxybenzoicacid and methyl red is used in a film 12 /p, in thickness, this film isrendered transparent by an applied field in the order of 40 kV/cm or avoltage in the order of 48 V On the other hand, if selenium is used asthe photoconductor, this having a resistivity in darkness in the sameorder O..cm) as that of the mixture of liquid crystal and dichroiccolorant, a film 8 of the same thickness as the film 9 will be used.Under these conditions, k, is approximately equal to l and the voltage Vis made equal to 50 V, this giving in the dark condition a voltage of 25V across the terminals of the film 9.

Then, the strength of the recording beam l2 is selected so that theresistivity of the photoconductor is approximately divided by 25, givingthe desired value for the field to be applied to the film 9.

In the case of certain liquid crystals, a current may flow between theelectrodes 7. However, this interferes with proper exploitation of thephenomenon involved and thus with the quality of the stored image. Toprevent this from happening, it is possible to interpose (FIG. 4) atransparent insulating layer or film 70, for example between the film 9and the corresponding electrode 7. For example, a film of mylar can beused which can also be employed in the construction of the electrode 7itself by metalizing the external face of the mylar film 70.

In FIG. 5, a complete embodiment of a storage device in accordance withthe invention has been shown.

Reference numerals which are the same as those used in FIG. 3, relate tothe same elements.

A voltage source 15 is connected across the terminals 11. This sourcecan produce a d.c. voltage or a rectangular waveform voltage if it isdesired to carry out erasing at regular intervals.

A light source 16, a lens 17 and a polarizer 18 which rectilinearlypolarizes the light perpendicularly to the plane of the figure,producethe desired light beam of wavelength A to illuminate the film 9. Thepolarizer 18 can also be placed against the right-hand electrode 7 inwhich case its orientation is immaterial.

The data to be stored and which are converted into optical form in thezone 21, are projected by the lens 22 onto the photoconductive film 8.

Thus, a light spot M produces an image at M, on the film 8. Thecorresponding spot M, on the film 9 then appears as a light spot. Evenif the light beam coming from M is cut, the beam coming from the source16 will continue to maintain the photoconductor in the conductivecondition at the point M Since the liquid crystal used should be in thenematic condition, it is necessary to maintain it within a certaintemperature range which defines this condition. For this purpose, it maybe necessary to provide a temperature control system 23 which has beenillustrated highly schematically here by a radiation heating device.Obviously, any other heating device could be provided.

Naturally, the device in accordance with the invention can be used forthe point by point recording of binary information and for theirstorage, or can equally well be employed for direct viewing as a storagescreen instead of a storage tube. The recorded image can then be thatproduced by a small cathode-ray tube whose screen is located at 21 andthis image can then be recorded in one go or in staggered fashion (forexample as in a sonar equipment).

What is claimed, is:

1. An optical information storing system comprising: a first constantthickness layer of a mixture of nematic liquid crystal and of a dichroicsubstance, said substance exhibiting at least one optical absorptionline with respect to rectilinearly polarized light; a second constantthickness layer of photoconductive material located against said firstlayer;two transparent electrodes for enclosing said layers ;first meansfor coupling said electrodes to a d.c. supply source optical means forprojecting said optical information onto said photoconductive layer andsecond means for illuminating substantially uniformly said first layerby a beam of polarized light comprising at least a light radiationhaving a wavelength substantially equal to the wavelength of saidabsorption line, said photoconductive material being sensitive toradiation from said optical information projecting means and from saidsecond means.

2. A system as claimed in claim 1 wherein said second means comprise asubstantially monochromatic light source and a polarizer for supplyingsaid beam with a rectilinear polarization in a direction parallel tosaid electrodes.

3. A system as claimed in claim 1 wherein at least one transparentinsulating layer is located between said electrodes.

4. A system as claimed in claim 2 wherein at least one transparentinsulating layer is located between said electrodes.

1. An optical information storing system comprising: a first constAntthickness layer of a mixture of nematic liquid crystal and of a dichroicsubstance, said substance exhibiting at least one optical absorptionline with respect to rectilinearly polarized light; a second constantthickness layer of photoconductive material located against said firstlayer;two transparent electrodes for enclosing said layers ;first meansfor coupling said electrodes to a d.c. supply source ; optical means forprojecting said optical information onto said photoconductive layer ;and second means for illuminating substantially uniformly said firstlayer by a beam of polarized light comprising at least a light radiationhaving a wavelength substantially equal to the wavelength of saidabsorption line, said photoconductive material being sensitive toradiation from said optical information projecting means and from saidsecond means.
 2. A system as claimed in claim 1 wherein said secondmeans comprise a substantially monochromatic light source and apolarizer for supplying said beam with a rectilinear polarization in adirection parallel to said electrodes.
 3. A system as claimed in claim 1wherein at least one transparent insulating layer is located betweensaid electrodes.
 4. A system as claimed in claim 2 wherein at least onetransparent insulating layer is located between said electrodes.